4sillypwr

overbore=more displacment. You are compressing more air into the same size combustion chamber. Someone correct me if I am wrong.

-David

bobski

that would be true if the head were just a flat plate on top of the pistons (which btw would result in a CR of around 37:1). i'm sure you know the head isn't flat, but has arched areas (for the valves and spark plug) with a specfic volume that doesn't change with the bore of the cylinder.
when you increase the bore of the cylinder, you increase the displacement of the engine slightly. the extra air has to go somewhere during the compression stroke, and last i checked, overbore A6s don't develop a 500+ psi ring of air around the rim of the piston - it contributes to an increase in overall compression.

4sillypwr

thanks for the backup. let me try to explain it like this. These are all just arbitrary numbers by the way. Its just the general idea. the area of a circle is pie x r^2 so.
stock bore 75mm. over bore of .5 mm. take the area of the overored cyl and subtract the area of the stock bore.

so the area of the top of the overbored cyl is 4476.97mm^2 and the stock bore is 4417.86mm^2

subtract and you get a ring of area 59.11mm^2.

Now for ***** and giggles assume the z6 piston hase a .5 mm piston to deck height. It's actually zero but that doesn't matter. multiply the area of the ring by the .5mm to get the volume of the area that you have added to the compression chamber.
59.11mm^2 x .5mm =29.555mm^3

Now that ring of overbore goes all the way down the cyl. So take the area of that ring and multiply it by the stroke of 90mm.
so 59.11mm^ x 90mm = 5319.99mm^3

Now all these numbers arnt exactly right on but the concept is the same.
when you over bore you are compressing 5319.99mm^3 into a 59.11mm^3 space. Its not exactly like this but the concept holds true. The z6 actually has zero piston the deck hieght and the overbore of the headgasket is already included in the stock compression ratio. In real life you are just compressing ~5320mm^3 into the same exact area you were with a stock bore.

-David

4sillypwr

Yeah I didn't figue the total volume displaced. just the small ring. like sitting a stock bore piston into an overbored cyl and measuring the area around it. then measuring that volume all the way down the cyl. Not accurate numbers but I hope it explains the concept.

-David